1. Field of the Invention
The present invention relates to a process for producing a Nd-Fe-B magnet in which the powder mixture mainly composed of anisotropic Nd-Fe-B magnet powder (hereinafter referred to as anisotropic magnet powder) and isotropic Nd-Fe-B magnet powder (hereinafter referred to as isotropic magnet powder) is compressed and heated by current supply so as to be fixed to a specific configuration.
2. Description of the Related Art
Conventionally, isotropic magnet powder can be obtained by the following process. Molten alloy having (Nd/Pr):(Fe/Co):B ratio of approximately 2:14:1 is melt-spun and suitably heat-treated at a temperature of not less than the crystallization temperature, as disclosed in, for example, "Producing of Neodymium-Iron-Boron Melt-Spun Ribbons to Fully Dense Magnet" IEEE T. MAG. Vol. MAG-21, No. 5 (1985) by R. W. Lee, et al. and "Rare Earth-Iron-Boron Materials; A New Era in Permanent Magnets" Ann. Rev. Sci. Vol. 16 (1986) by J. F. Herbest. In the isotropic magnet powder there occurs such fine structure that Nd.sub.2 Fe.sub.14 B phase with a size approximately from 20 to 200 nm is dispersed at random in an amorphous Fe phase, thereby developing the coercive force (Hcj).
The state of a material obtained by the above melt-spinning is restricted to a powder. No matter what kind of method is used, in order to obtain a magnet of a specific configuration generally used, a technique is required to fix the isotropic magnet powder into a specific configuration. In the field of powder metallurgy, sintering under an atmospheric pressure is a basic technique for fixing a powder. Since it is necessary to heat the powder at a far higher temperature than the crystallization temperature in this method, the Nd.sub.2 Fe.sub.14 B phase is excessively grown, thus reducing the coercive force (Hcj).
In order to overcome the above drawback, as disclosed in, for example, U.S. Pat. Nos. 4,689,163, 4,981,635 and 5,100,604, isotropic magnet powder is fixed to a specific configuration by generally using a different kind of material, for example, a resin. The obtained magnet has been developed and put into practice. The magnet fixed by such a resin has a maximum energy product (BH)max of up to 9 MGOe.
When the isotropic magnet powder is hot die-up-setting worked, as disclosed in the theses by R. W. Lee and J. F. Herbest, the crystal orientation of the Nd.sub.2 Fe.sub.14 B phase is changed, thereby showing remarkable anisotropic characteristics and thus reaching the (BH)max of up to 40 MGOe. However, in the hot die-up-setting method, it is difficult to directly obtain a specifically-configured magnet in which the practical near-net-shape is ensured. It is also difficult to grind the magnet.
In view of this background, an applied technique has been developed and examined as follows. As disclosed in, for example, "Pulverizing Anisotropic Rapidly Solidified Nd-Fe-B Materials for Bonded Magnet" J. Appl. Phys. 70(10), 15(1991) by M. Doser, V. Panchnathan, after the isotropic magnet powder is hot die-up-setting worked, it is pulverized so as to produce anisotropic magnet powder which is orientated in a magnetic field and then fixed by a different kind of material, such as a resin. The resultant magnet has a (BH)max of about 15 MGOe higher than a magnet obtained by fixing isotropic magnet powder by a resin.
In order to target a high (BH)max for the magnet obtained by fixing the anisotropic magnet powder by a different kind of material, the anisotropic magnet powder is required to possess both strong anisotropic characteristics and high (BH)max. It is also important to obtain a technique of fixing the anisotropic magnet powder with the particular orientation while maintaining the orientation and compressing a mass of the powder to reduce gaps, thus increasing the volume fraction of the anisotropic magnet powder in the magnet.
It is relatively easy to magnetically orientate the anisotropic magnet powder by use of a magnetic field. However, in general, it is necessary to compress the anisotropic magnet powder with high stress, for example, a few tons/cm.sup.2 at approximately room temperature with a view to increasing the volume fraction of the anisotropic magnet powder in the specifically-configured magnet. The anisotropic magnet powder has great inter-grain friction when it is compressed at room temperature, and thus, it is partially destroyed when it is displaced to fill gaps and when it becomes densely-packed. The coercive force Hcj of the anisotropic magnet powder greatly depends on the grain size; for example, the fine powder with a grain size of 50 .mu.m or less lowers Hcj. Also, the magnet powder destroyed by compression disturbs the orientation, thus failing to obtain a high (BH)max. Further, the volume fraction of the anisotropic magnet powder in the magnet is limited to approximately 80 vol %. As will be seen from the above problems, such technique is coming to be greatly demanded that, while the anisotropic magnet powder is densely packed, the damage of the powder is minimized to suppress the disturbance of the orientation and the volume fraction of the anisotropic magnet powder in the magnet is made to be increased.